See bug 1405548 for some context. With the Gecko style backend we assume that all changes to CSS animations should not generate subsequent restyles and we have various methods that try (but often fail) to ensure this. Methods like XXXNoUpdate and so on. When using the Servo style backend, however, we actually want these changes to trigger restyles so that we will perform a second animation traversal. We'll still need to keep PlayFromStyle() and PauseFromStyle() at least since these do other things apart from just avoiding posting a restyle, but there are probably a few methods we can drop including the extra machinery that bug 1405548 is likely to add.